FYI, the cats are not literally almost liquid in body composition.
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I suspect its because it makes for a catchy headline.
The contrast with dogs in the introduction is instructive: dogs tend to hunt over open fields rather than chasing prey into narrow dens, so it makes sense they would tend to make conservative eyeball judgments about whether they can fit into certain spaces. But cats will try to corner their prey in a tunnel/etc, so they have good reason to rely more on touch and experimentation ("ecologically-valid strategy").